1. Field of the Invention
This invention relates to a combination utility module for monitoring the three-dimensional configuration of a streamer cable and for retrieving damaged cable portions in the event of a catastrophe such as that which occurs when the cable exceeds a preselected depth or is rent asunder by a passing ship.
2. Description of Related Art
In marine seismic exploration, a ship tows a long streamer cable at a desired depth such as 15-20 meters below the water surface. The streamer cable contains acoustic receivers distributed therealong at desired spaced-apart intervals. A sound source periodically generates an acoustic wavefield which propagates through the water and into the earth beneath to insonify subsurface earth layers. The wavefield is reflected from the respective earth layers back towards the water surface where the reflected seismic waves are detected by the acoustic receivers. The acoustic or seismic receivers may be hydrophones, geophones, accelerometers or combinations thereof. The receivers convert the mechanical seismic waves to electrical signals which are sent back to the ship, through suitable transmission means in the cable, for archival recording and later processing.
Typically, a seismic cable may be 3000 to 4000 meters or more long. It may contain as many as a thousand individual data receiver channels. To avoid the need for thousands of separate wire lines, digital modules are mounted in the seismic streamer cable at intervals of 100 to 150 meters. Each module receives analog data from respective ones of a plurality of receivers, digitizes the analog signals and multiplexes the resulting digital signals back to the ship over a single wide-band transmission link of any desired type. Electrical power for activating the digital modules is transmitted from the ship over a suitable power line in the cable for distribution to the respective modules.
A seismic streamer cable is usually made up of many sections, each 100 to 150 meters long. A section consists of a PVC jacket about 5 cm in diameter. The seismic receivers, data-communication channels, power lines, command lines and the like are mounted inside the cable. The digital modules are usually designed to mechanically and electrically interconnect together respective ones of the plurality of individual cable sections as well as to pre-process the received seismic signals. Because the above-described hardware is heavy, the PVC jacket is filled with a lightweight fluid such as odorless kerosene to render the cable sections neutrally buoyant with respect to the average sea water density.
A streamer cable is long and flexible. Its configuration in three axes must be known. Although the location of the head end of the cable is known, the effects of wind and sea currents may cause other parts of the cable to wander away from a desired line of survey by a substantial distance. Thermoclines and local changes in seawater density may cause the cable to erratically sink or rise to undesired depths. Therefore, configuration-monitoring and controlling means are provided in the cable.
Three-dimensional cable configuration may be monitored with the aid of an instrumented tail buoy in combination with a number of three-axis sensing modules such as disclosed in U.S. Pat. No. 5,105,548 for an Apparatus for Determining Azimuth, Pitch and Roll, issued Apr. 21, 1992 to John T. Fowler and a Digicourse advertisement taken from the September 1993 issue of The Leading Edge. A tail buoy may include some form of geo-location equipment such as a GPS receiver. The sensing modules may include digital compasses for measuring local cable orientation and pressure-actuated depth sensors for measuring and reporting the local cable depth. The modules are mounted in the cable at selected intervals therealong such that the sinuosity of the cable can be accurately tracked.
The cable depth may be controlled by means of a module having pressure-actuated or servo-actuated diving planes. A number of control modules, also commonly referred to as Birds, are rotatably secured to the seismic cable at intervals therealong. The birds each include comparator means that may be preset to cause the bird to fly at a preselected depth. One such device is disclosed in U.S. Pat. No. 3,931,608, issued Jan. 06, 1976, to Jimmy R, Cole. The depth controller may of course be combined with the cable-configuration monitor as above described. The '608 patent also teaches a method for securing the modules to the cable using rotating attachment rings.
Navigation information from the tail buoy, the compasses, the depth sensors and commands to the depth controllers is transmitted bi-directionally between the towing ship and the cable-mounted sensors over a dedicated communications line. Information is typically encoded and transmitted using phase-shift-keying (FSK) over a 25-kHz carrier. Instrumentation signals from the respective modules are coupled into the communications line with the aid of an induction loop in the module that is associated with a mating induction coil mounted in the cable near the module attachment rings.
A seismic streamer cable complete with all of the accessories as enumerated is very expensive, on the order of a million dollars or more. In the event of a catastrophe such as the cable being snagged on an obstruction or cut by the screws of a passing ship, or damaged by reason of an excessive depth, it is prudent and economical to recover the remanent(s) of the cable for repair. Since the cable is normally neutrally buoyant, it does not necessarily surface after being cut. In fact when it has been cut, the jacket of the cut section loses its ballast fluid so that the cable remanent sinks.
There are devices available commercially for retrieving a damaged cable section. One such device is described in U.S. Pat. No. 4,541,079, issued Sep. 10, 1985 to Ben B, Thigpen. Here, a CO.sub.2 cartridge is provided in selected cable sections. If the cable becomes damaged, an operator on the towing ship causes an alarm signal to be transmitted through the water to a responder associated with the damaged section and to other nearby sections. Upon receipt of the alarm signal, the cartridge is fired to release the gas and inflate the cable jackets of the one or more nearby undamaged sections. The inflated section then floats to the surface. The problem with that system is that it requires operator input. If the operator is unaware of the existence of cable damage, the damaged cable is lost and gone forever.
U.S. Pat. No. 4,823,325, issued Apr. 18, 1989 to Jimmy R. Cole Jr, teaches a cable retriever that uses an inflatable bag to lift the streamer to the water surface. An acoustical signal or a pressure switch activates the device and inflates the bag. An electric drill is used to puncture the frangible disk that seals the CO.sub.2 cylinder. Although the triggering device is self-actuating when the cable exceeds a safe depth limit, in shallow water, the cable my never exceed the depth limit so that the retriever would not activate. Furthermore, the need for an electric drill complicates the system and adds weight. Additionally, the steel CO.sub.2 bottle interferes with magnetic sensing devices.
The three types of devices that were discussed above are customarily hung on the seismic streamer cable at various locations therealong. Because the devices are heavy, each must include its own ballast. A plurality of modules hanging outboard of the cable create a clutter of hardware that increases the required towing force and substantially increases the ambient tow noise that is superimposed upon the relatively weak seismic signals.
There is a need for a single compact utility module for use with a seismic cable that includes the combined features of a multi-axis configuration-monitoring system, a depth controller, a module-floatation means and a self-activating cable retriever. The utility module including the cable retriever must be free from stray magnetic fields that would interfere with the magnetometer-type sensing device that is incorporated with the configuration-monitoring system. Once initialized, the damage-control system operation must be independent of any further need for operator input.